The answer is clearly Rs. 1000. The other shopkeeper has no bearing on the answer, he's been mentioned only to confuse the reader. Think of it as an equation : Profit of the lady = Loss of the shopkeeper. Since the lady made a profit of Rs. 1000, i.e 200 in goods and 800 in cash, therefore, the loss of the shopkeeper must also be Rs. 1000.
A Lady buys goods of Rs.200 & Pays a Rs.1000 note to shopkeeper , he gets the change from the Next shop & & keeps Rs.200 for himself & Returns Rs.800 to the lady ..
After some time, The next shop owner Returns the Rs.1000 note Saying that its " Duplicate" & Takes the money Back ..
Now the Question is How much Loss did the shpokeeper Suffer ?
The actual loss is Rs 1200 less the projected profit.